Fix race in pselect replacement code.

Fix race in pselect replacement code.

On the second and subsequent calls to pselect the notify_pipe was not
added to the select readset, opening up a race that om G. Christensen
discovered on multiprocessor Solaris <=9 systems.

Also reinitialize notify_pipe if the pid changes.  This will prevent a
parent and child from using the same FD, although this is not an issue
in the current structure it might be in future.